Indicators of aortic blood flow, average heart rate, heart rate variability and beat-to-beat interval can be used to infer levels of sympathetic and parasympathetic nervous system activity, collectively referred to as autonomic tone. An individual's level of autonomic tone is associated with a level of arousal. When used in the context of a vehicle, estimation of autonomic tone of a driver can indicate suboptimal levels of arousal and therefore can be used to detect driver impairment.
Different vehicle interfaces exist to determine autonomic tone of a driver in a vehicle. For example, an interface can acquire different biological signals (e.g., indicating aortic blood flow, average heart rate, heart rate variability and beat-to-beat interval.) from a driver and analyze the biological signals to determine an estimation of autonomic tone. The vehicle environment, specifically, noise and vibrations from engine idling, road travel, among other sources, can interfere with the acquisition and analysis of biological signals in the vehicle and therefore influence the estimation of autonomic tone.